Through my time in EDT online groups I have come across the 2 most extreme EDTs I have ever heard of.
Usually the "extreme" part of an EDT is said to come from the low cost of the flights etc
But I'd say these are extreme in another way.
Ive heard of someone who has done Manchester to Istanbul and one of the London area airports to New York.
I doubt either were cheap in the flight prices department but extreme in pushing the furthest limits of what you could actually do for a day trip.
You must have missed the first page of this thread where someone mentioned a (futile) day trip from the UK to Los Angeles for work, as well as Concorde to New York and back, which I believe was quite common back in the day when financial and legal firms needed wet ink signatures on contracts or other urgent physical documents transferred internally.
You must also have missed my post about my "day trip" to New Zealand, well technically more than a day as I spent around 28 hours there but only slept upright on a chair in the airport twice (arrived 1am, waited until public transport started at 6am, then returned on the last bus around midnight and departed 6am). As I stopped over in several other places for less than a day each, I was away from the UK for over a week on that trip.
That post attracted a reply about someone who went to NZ for a job interview and came straight back, with no luggage at all, which I think is the winner.
Istanbul is probably the sweet spot of returning within 24 hours while having a meaningful time at the destination, because of the existence of an "overnight" flight (in the case of MAN, it is currently TK1916 departing 11pm arriving 5am so you get about 12 hours there before having to return on TK1915 at 7pm arriving 10pm). Cyprus also looks possible, and I guess Tel Aviv if you are so inclined, but the Israeli security services would not like that.
Even without Concorde, North America is not difficult at all because of how the flight schedules work from Europe. You can easily return to the UK within 24 hours of your departure, although to maximise your time at there it would be better to make it into a 1½ day trip. The same applies to much of the Middle East, with so many flights at all hours of the day to Dubai and Doha.
London to Edinburgh (typically yes, even if you went by car, train or national express coach).
Train is easy, coach you would probably go overnight both ways (I did once) so stretching the definition of a day trip. Car I would say no unless "you" are not the driver or there are 3+ rotating drivers.
My cousin claims that in the 1980s he drove from Toronto to Memphis without stopping except at traffic lights and at the border - apparently he didn't need to get out of the car once.
One night stays are often considered to count.
Well if that counts then the number of "day trips" I've done more than triples. Starting from London - by car to Aberdeen and Glasgow multiple times, by plane and train to most EU countries, and to Asia and Australia several times (actually my NZ trip was a tag on to an Australia trip when I realised it would cost a negligible amount of extra points to redeem).